1. Field of the Invention
The present invention relates to a camera system and, more particularly, to an automatic focus control in response to the focus detection through an objective lens.
2. Description of the Prior Art
Conventionally, there has already been proposed a focus detection device which is so arranged that two images are formed by again bringing into focus, light bundles respectively having passed through corresponding first and second regions of an objective lens which are in a symmetrical relation with respect to an optical axis of said objective lens, and by finding the mutual positional relation of these two images, a degree of out of focus at the image forming position with respect to a predetermined focal plane and a direction thereof (whether the image forming position is at the front side or at the rear side of the predetermined focal plane, i.e., the focusing is in a front-focus or rear-focus) are to be obtained.
The optical system of the focus detection device as referred to above has a construction as shown in FIG. 19, which generally includes a condenser lens 6 disposed in a predetermined focal plane 4 behind an objective lens 2 or in a position further behind the focal plane 4, and re-focusing lenses 8 and 10 provided behind the condenser lens 6, while line sensor having, for example, CCD (charge coupled device) as light receiving device is disposed on the focusing planes of said respective re-focusing lenses 8 and 10 as at 12. The images formed on the line sensor approach the optical axis 14 so as to come close to each other in the case of the so-called front-focus condition in which the image of an object to be brought into focus is formed at a position before the predetermined focal plane 4, while on the contrary, they are respectively spaced away from the optical axis 14 in the case of the rear-focus condition. When the images are brought into focus, the interval between the corresponding two points of the two images reaches a specific distance as defined by the construction of the optical system for the focus detection device. Accordingly, in terms of principle, the state of focusing is to be found through detection of the interval between the two images.
In the focus detection device as described above, however, in the case where the focus detecting functions are repeatedly effected by a plurality of times with respect to the same object to be photographed at a predetermined distance, if the object to be photographed is altered in its position within a plane perpendicular to the direction for detecting the distance or the camera is subjected to a minor vibration due to wobbling or shaking during holding thereof, the results of detection are not necessarily coincident, thus being distributed through a slight scattering with respect to a certain value. In other words, there has been such a disadvantage that an accurate focus detecting information can not be obtained based on the detecting function only by one time.
Besides the scattering arising from the non-uniformity in the characteristics and arrangement of respective elements in a light receiving device composed of a large number of light receiving elements, and also, arising from instability of processing circuits, the scattering as referred to above may be attributable to the fact that in a measurement of illumination distribution (equivalent to the distribution of brightness for the object to be photographed) on the light receiving surface, spatial frequency characteristics possessed by the light receiving elements are determined by the pitch of the group of elements arranged in the array, thus it being made impossible to effect correct measurements with respect to the frequency component higher than the spatial frequency to be determined by the Nyquist sampling law, or such scattering may be ascribed to the fact that since the brightness distribution of the object to be photographed is discontinuously measured by the presence of insensitive regions between the elements, it is impossible to detect any possible brightness variation in such insensitive regions.
Accordingly, if the object to be photographed is altered in its position in a plane perpendicular to the detecting direction, the pattern of the image of the object projected onto the light receiving surface is altered, with a relative alteration of the sampling position of the object image on the light receiving array, thus giving rise to such a phenomenon that the results of detection are not coincident by the above causes in the elements, and therefore, if similar measurements are repeated, the detection results are distributed through a slight scattering with respect to a certain value. Accordingly, even if the objective lens is adjusted for the focusing based on a defocus amount to be obtained in one detecting function, accuracy for the adjustment can not be guaranteed.
Such being the case, there has been proposed in Japanese Patent Laid-Open Application (Tokkaisho) No. 56-78811, a device which is so arranged that detection of focus condition is effected by a plurality of times to obtain data for a plurality of defocus amounts, thereby to obtain an average value thereof, and the focus adjustment of the objective lens is effected based on said average value. However, in the known device as described above, a quick focus adjustment can not be effected, since the arrangement is so made that the detecting function is repeated, with the objective lens being held stationary so as to start driving of the objective lens after the average value has been obtained.
Therefore, there has also been proposed U.S. Pat. Nos. 4,445,761 and 4,492,448 assigned to the same assignee as that in the present invention, a focus adjusting device which is so arranged that the focus detection is repeatedly effected during displacement of the objective lens towards the in-focus position, while the amount of displacement of the objective lens is detected through employment of an encoder in that case, and thus, the data for the respective defocus amounts taken at separate positions during the displacement of the objective lens are subjected to correction by the amount of displacement of the objective lens, thereby to convert the respective data into data which may be regarded as those taken, with one position set as a reference position so as to obtain the average value from the data after the conversion. In the above proposed device, the average value is calculated, with the same weight imparted to the respective data.
Meanwhile, in the case where a lens such as a telephoto lens with a long focal length, etc. in which the defocus amount can be increased, is to be employed, there may arise a region which is out of a range associated with the defocus amount detection of the focus detection device. In the region as described above, the focus detection device undesirably judges that the focus detection is impossible, since contrast of the object to be photographed is too low. In order to overcome the disadvantage as described above, there has further been proposed in Japanese Patent Laid-Open Application (Tokkaisho) No. 59-182441 also assigned to the same assignee as that of the present invention, a focus detection in which the case as referred to above, the lens is displaced by neglecting the defocus amount already obtained, to find out a position at high contrast, and if such a position has been found, the displacement of the objective lens is suspended for effecting a subsequent focus detection. In the above practice, however, the function is undesirable such that the lens is subjected to the scanning driving during low contrast, and upon finding a position at high contrast, the lens is necessarily stopped once and again displaced towards the in-focus position. In other words, in the above function, there is such a drawback that smooth and quick focus adjustment can not be effected at all times.
However, if an image sensor of a charge accumulation type such as CCD (charge coupled device) is employed for the focus detecting line sensor of an automatic focus control arrangement, the integrating time thereof becomes long as the brightness of the object is reduced. Accordingly, the lens displacing amount during the CCD integration varies to correspond to the brightness of the object, while owing to the fact that the image of the object to be photographed during the integration varies according to the brightness of the object, the error of the defocus amount itself as calculated is also altered. Meanwhile, if a practice for integrating the object image during displacement of the objective lens is adopted, the state is the same for the CCD as in the integration of a moving object even when the object remains stationary in the case where a long integrating time is required, and errors tend to be involved in the calculation of the defocus amount, with such a tendency being further increased if the object to be photographed is moving.
Moreover, in the conventional automatic focus control device, if the contrast of the object to be photographed is too low, there is such a disadvantage that the focus detection becomes impossible or detecting accuracy is extremely deteriorated.
More specifically, if the CCD integration is effected while the objective lens is being displaced when the object to be photographed is dark, the resultant accuracy of the defocus amount tends to be deteriorated, and there may possible arise such a case that the stopping position of the lens is not fixedly determined due to scattering of the results, thus requiring a long time until arrival at the in-focus condition. On the other hand, if the object to be photographed is bright, deterioration in the accuracy hardly takes place owing to a short integrating time, even when the CCD integration is effected, with the objective lens being displaced.